The Complex Genetic Basis of Resistance toBacillus thuringiensisToxin in Insects

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Abstract
Recent studies have shown that the genetic basis of resistance to Bacillus thuringiensis (Bt) toxin in insects is far from simple. In this respect, it is similar to resistance to chemical insecticides, which is conferred by multiple physiological mechanisms under independent genetic control. Several lines of evidence for a complex genetic basis for resistance in tobacco budworm Heliothis virescens and diamondback moth Plutella xylostella are reviewed. This suggests that a genetic approach based on linkage mapping, capable of simultaneously analyzing multiple physiological mechanisms, will be very useful in dissecting the complex trait of resistance. In Heliothis, the existence of separate, independently assorting resistance genes has already been confirmed by linkage analysis with marker loci. This approach assists in the measurement of the relative potency of different resistance genes present in the same strain. Additionally, it facilitates comparative studies of the independent acquisition of homologous resistance mechanisms by different species. Perhaps most important from a resistance management standpoint, it directly addresses the question of whether cross‐resistance is due to separate loci or the same locus. Differing properties of the diverse physiological mechanisms capable of conferring resistance yield different outcomes when contrasting scenarios of transgene toxin deployment are considered. These genetic issues have important ecological implications because they affect the rate of resistance evolution in the pests, an understanding of which is essential to the development of sensible deployment strategies of transgenic crop plants to maximize their efficacy over the long term.